An ophthalmic lens blank generally has a first face with a pre-determined curvature and a second face, opposite the first face on which a desired surface contour is generated by a machining process. The overall process is generally referred to as “lens surfacing” and the overall object is to yield a finished spectacle lens L (shown in section in FIG. 6) so that the curvature of the first (in this instance convex) face cx and the curvature of the machined second (in this instance concave) face cc cooperate to yield desired optical properties according to the prescription. In addition to this the first and/or second faces cx, cc of the lens L are usually coated, cf. FIG. 7 illustrating in an enlarged view of detail VII in FIG. 6 an example for such coating, to provide the finished spectacle lens L with an enhanced ability to resist scratching (by use of a “hard coating” HC), with a low residual reflection and/or a desired color (by use of an “antireflection coating” AR), and/or with certain surface properties such as hydrophobic, oleophobic and/or dirt repelling properties (by use of a “top coating” TC). Usually also a further machining process takes place (the so-called “edging”), the aim of which is to finish-machine the edge E of the spectacle lens L in such a way that the spectacle lens L may be inserted into a spectacle frame. In all these process steps the spectacle lens (blank) L must reliably be held in the machining machine(s) and coating apparatus, respectively.
To this end, U.S. Pat. No. 8,905,388 B2 of the present applicant discloses a special block piece for holding the spectacle lens (blank) throughout the above mentioned processing steps. To be more precise, prior to the processing the lens blank with its first face cx is “blocked” with the aid of a blocking material, for instance a UV or visible light (VIS) radiation curable adhesive composition, on a workpiece mounting face of this block piece, where it remains throughout the processing of in particular the second face cc, including surfacing and thin film coating under vacuum conditions on one and the same block piece, until the processed spectacle lens L is “deblocked” again from the block piece. So as to further minimize the production efforts in the prescription workshop it has further been proposed in this prior art to fully finish the first, blocking face cx of the lens blank prior to the blocking step, including hard coating HC, antireflection coating AR and, if applicable, top coating TC.
The hydrophobic and/or oil-repellent anti-stain top coatings TC, however, most often consist of fluorosilane-type materials that reduce the surface energy so as to prevent the adhesion of greasy stains which are thereby easier to remove. One of the problems associated with such top coating TC is that it may achieve such an efficiency that the adhesion at the interface between the blocking material and the first, blocking face cx of the lens blank is thereby altered. As a result there is a risk that the adhesive connection between the lens blank and the block piece cannot sufficiently withstand the forces that are exerted on the lens blank during the machining thereof. At the worst, the lens blank comes off the block piece and is ruined.
In order to address such problem, it has been proposed already to coat onto a lens blank having an outer hydrophobic and/or oil-repellent surface coating a temporary protective layer comprised of a metallic fluoride, in particular MgF2, and imparting to the lens blank a surface energy at least equal to 15 mJ/m2 so as to achieve sufficient adhesion at the interface between a holding pad and the lens blank for holding pads conventionally used in the technical field to hold the lens blank on a block piece during the edging step. Examples of this method are disclosed in U.S. Publication 2003/0049370 A1 and U.S. Publication 2006/0246278 A1.
This known MgF2 “grip system” functions well as long as it is used for traditional processes in which the blocking and edging steps take place just after the coating steps. However, as a solution for a continuous on-block manufacturing process as disclosed in U.S. Pat. No. 8,905,388 B2 it works in a very limited way only, mainly because of losing surface energy performance by time, as tests have shown.
In the latter process, surface energies of 35 mJ/m2 or higher are necessary in order to provide sufficient adhesion for the surfacing steps. In addition to this, blocking of the cx coated blanks usually takes place only after a certain storage time which may amount up to several weeks in the longest case.
As an example, in the conducted tests a temporary protective layer comprised of MgF2 and having a thickness of 15 nm showed a surface energy of 39 mJ/m2 after a period of one day from coating, but 19 mJ/m2 only at the end of one month after coating. A similar temporary protective layer with a thickness of 90 nm had a surface energy of 62 mJ/m2 one day after coating, and of 35 mJ/m2 after expiry of one month. So the 15 nm layer cannot be used as “grip measure” in the continuous on-block manufacturing process at issue since its surface energy is too low after storage. From the surface energy point of view, the 90 nm layer could only just be used in the process at issue. However, especially in case of thermoelastic substrate materials such as CR 39, the stress to the antireflection coating on the first blocking face of the lens blank would be rather high during the surfacing steps because of the thickness of the 90 nm layer, possibly causing damage to the antireflection coating.
What is desired starting from the generic prior art according to U.S. Publication 2003/0049370 A1 is to provide a lens blank which already has a fully finished first face including in particular a hydrophobic, oleophobic and/or dirt repelling top coating covered by a temporary grip coating, that overcomes the drawbacks previously mentioned and enables in particular spectacle lenses with high optical qualities to be produced in a continuous on-block manufacturing process as disclosed in U.S. Pat. No. 8,905,388 B2, without the risk that the lens blank unintentionally comes off the block piece during the processing steps. The object of the invention further encompasses the provision of a combination of a suited lens blank and a block piece for holding it, of a method of blocking such lens blank, the use of the latter in a method for manufacturing spectacle lenses according to a prescription, and of such method.